Advanced progress of vestibular compensation in vestibular neural networks

Abstract Vestibular compensation is the natural process of recovery that occurs with acute peripheral vestibular lesion. Here, we summarize the current understanding of the mechanisms underlying vestibular compensation, focusing on the role of the medial vestibular nucleus (MVN), the central hub of the vestibular system, and its associated neural networks. The disruption of neural activity balance between the bilateral MVNs underlies the vestibular symptoms after unilateral vestibular damage, and this balance disruption can be partially reversed by the mutual inhibitory projections between the bilateral MVNs, and their top‐down regulation by other brain regions via different neurotransmitters. However, the detailed mechanism of how MVN is involved in vestibular compensation and regulated remains largely unknown. A deeper understanding of the vestibular neural network and the neurotransmitter systems involved in vestibular compensation holds promise for improving treatment outcomes and developing more effective interventions for vestibular disorders.


| INTRODUC TI ON
[3][4] Interestingly, these symptoms caused by unilateral vestibular injury can partially alleviate over time through central nervous system (CNS) plasticity, called vestibular compensation, [5][6][7][8][9][10] which can be promoted by medications and vestibular rehabilitation.The two medial vestibular nuclei (MVN) on either side of the brainstem are considered the hub of the vestibular system and the critical brain region for vestibular compensation. 10They are mutually projected to each other via GABAergic and glycinergic neurons, 11 integrating the peripheral vestibular input and top-down regulation from other brain regions such as the cortex, 10 cerebellum, 12 thalamus and hypothalamus. 13,14llowing unilateral vestibular damage, neurons in the ipsilateral vestibular nucleus become sedate due to reduced input from the vestibular system; in contrast, neurons in the contralateral vestibular nucleus become more active, which might be due to the reduced inhibitory input from the ipsilateral vestibular nucleus (Figure 1). 11This activity imbalance between both sides of the vestibular nucleus is believed to be underlying the vestibular syndromes, 15 which can be partially reversed during vestibular compensation. 16,17Understanding this endogenous recovery mechanism will be beneficial for developing new strategies for vestibular disorder treatment.It will also be fruitful for the investigation of CNS postlesional plasticity, the ability of CNS to compensate for lesion-impaired functions, including movement, cognition, and behavior.In this review, we will focus on the vestibular compensation mechanism of MVN and its related regulation neural networks to address the question: after unilateral vestibular injury, how does MVN integrate signals from the whole brain neural network and regulate vestibular compensation to facilitate recovery?

| THE S TRUC TURE AND FUN C TI ON OF THE VE S TIBUL AR N UCLEI
The vestibular nuclei are located in the brainstem, the lateral walls of the fourth ventricle, extending towards the junction of the medulla and the cerebellum. 18,19The vestibular nuclei receive most of the peripheral vestibular system projection to the CNS, and integrate multiple sensory inputs, orchestrating brain regions to control visual fixation, balance, and spatial orientation. 20,21Each side of the vestibular nucleus is composed of the superior, spinal, medial, and lateral vestibular nucleus 18,19 ; among these subnuclei, MVN is the largest and most widely studied region, 22 which has different functions longitudinally, caudal MVN neurons are the input source to ipsilateral flocculus, while the rostral MVN receives input from ipsilateral flocculus Purkinje cells. 22

| THE CELL T YPE S OF THE VE S TIBUL AR N UCLEI
According to cell size, vestibular nuclei cells can be divided into magnocellular and parvocellular cells.The magnocellular cells are located in the external region of vestibular nuclei and are mainly composed of multipolar glutaminergic neurons.4][25] GABAergic neurons in vestibular nuclei were divided into five groups 26 : The first group of GABAergic neurons forms associational fibers between the bilateral vestibular nuclei.The second, third, and fourth groups of GABAergic neurons project to the oculomotor nuclei, the inferior olive nucleus, and the spinal motor neurons, respectively.The fifth group is comprised of local interneurons in the MVN.

| THE E XCITAB ILIT Y OF VE S TIBUL AR NUCLEI NEURONS DURING VE S TIBUL AR COMPENSATION
After the unilateral vestibular injury, glutamate concentration decreased during the first 2 days on the ipsilateral side of MVN, 27 consistent with the observation that the primary vestibular fibers gradually degrade due to the deafferentation peripheral vestibular system. 28Nevertheless, the heightened frequency of excitatory postsynaptic currents (EPSC) implies an augmented discharge from excitatory projections from other brain regions, which might promote the restoration of ipsilateral MVN excitability and the neural activity balance of the bilateral MVN. 29 the contrary, the quantity of GABAergic terminals increased within the first 3 days after injury; in consistency, the inhibitory postsynaptic currents (IPSC) increased in the ipsilateral MVN. 30,31is enhanced inhibitory input may originate from GABAergic neurons in the contralateral vestibular nucleus 31 and local GABAergic neurons in the ipsilateral MVN. 32,33Three days after the unilateral vestibular injury, IPSC frequency gradually declines, accompanied by an increase in the frequency of EPSCs, restoring the neural activity balance in both vestibular nuclei. 34e rebalancing of firing activity in neurons of bilateral MVN is a result of the combined effect of these excitatory and inhibitory events, while the regulation of inhibitory transmission may be dominant. 10

| COMMISSUR AL INHIB ITORY SYS TEM
The projections between the bilateral vestibular nuclei are mainly inhibitory GABAergic transmission, mediated by GABA A receptors rather than GABA B receptors, constituting the commissural inhibitory system. 15When the unilateral vestibular is damaged, the reduced neural activity of the ipsilateral MVN results from both the loss of peripheral vestibular input and the contralateral MVN innervation The mechanism of vestibular compensation. 11The MVN receive afferent signals from the peripheral vestibular system and other brain regions, such as the cerebellum, the neuronal activity is balanced between the bilateral vestibular nuclei.After the unilateral peripheral injury, this loss of afferent leads to down-regulation of neuronal activity in the ipsilateral vestibular nucleus, while up-regulation in the contralateral side.This imbalance is believed leading to the symptoms of vestibular disorders.CNS can partially rebalance the neural activity between bilateral MVN, and reverse the vestibular symptoms.In contrast to the ipsilateral side, the contralateral MVN neurons become more active, which might be due to the reduced inhibitory input from the ipsilateral vestibular nucleus.Besides the inhibitory commissural projections between the bilateral MVN, the cerebellum and other brain region inputs are also critical for vestibular compensation.Green dashed line with arrowheads: Excitatory transmission; Brown dashed line with arrowheads: Inhibitory transmission.CNS, Central nervous system; MVN, Medial Vestibular Nucleus.
through the commissural inhibition system. 11,32,35This long-distance inhibitory projection from the contralateral vestibular nucleus likely plays a pivotal role, 10 as fewer MVN neurons are silenced, and the firing frequency of neurons is less reduced after bilateral peripheral damage, compared to unilateral damage.Importantly, no neural activity imbalance is observed in the bilateral vestibular nucleus, and the animals did not display any asymmetric postural behavior after bilateral vestibular damage. 36Consistent with these observations, although GABA levels increased in the ipsilateral vestibular nucleus, several days after unilateral vestibular damage, GABA levels returned to normal. 32In the contralateral vestibular nucleus, GABA levels decreased significantly, 37 and this rebalance of commissural inhibition is synchronized with the recovery of behavior. 32All the above findings collectively suggest that the commissural inhibitory system is critical in vestibular compensation.

| VE S TIBUL AR NEUR AL NE T WORK AND NEUROTR AN S MIT TER S DURING THE VE S TIBUL AR COMPENSATION
Besides the local circuitry in MVN and the commissural inhibitory system between the bilateral MVN, other brain regions also play a critical role in vestibular compensation, probably via modulating MVN neurons.Recently, significant advances have been made in elucidating the brain regions targeting MVN GABAergic and glutamatergic neurons (Table 1).More than 50 upstream nuclei were identified for MVN GABAergic and glutamatergic neurons, distributed mainly in the hindbrain, including the medulla, pons, midbrain, and cerebellum. 14,38,391][42] MVN GABAergic neurons received more inputs than glutamatergic neurons (Table 2) from distant brain regions, including the thalamus, hypothalamus and cortex.4][45][46] Inhibitory projections of the cerebellar lobules mainly project to the glutamatergic neurons of the MVN (Table 3). 47The functions of these upstream nuclei relate to balance maintenance, emotion control, and circadian rhythm regulation, suggesting the comorbid of vestibular disorders with anxiety/depression and sleep disorder, and the treatments aimed at these brain regions may aid in the promotion of vestibular compensation.
MVN receives inputs of neurotransmitters from different brain regions and integrates them to regulate postural control. 480][51][52][53][54] Here, we will discuss these neurotransmitters and their related brain regions.

| Cerebellum and GABA
6][57] The cerebellum has three lobes from the rostral to the caudal: the anterior, posterior, and flocculonodular lobes. 59][60] This region is one of the primary modulators of vestibular nuclei, and its deficit induces balance and gait disturbances. 61,623][74][75][76][77][78] When the climbing fibers inputs to the flocculus are disrupted, vestibular compensation is delayed, 72,79,80 suggesting the involvement of the flocculus in vestibular compensation.After the unilateral vestibular loss, a series of molecular changes in the flocculus are observed. 68,76

TA B L E 1
The distribution of brain regions that innervate both GABAergic and glutamatergic neurons in MVN. 38,39rain-derived neurotrophic factor (BDNF) upregulation may reduce the inhibitory effects of the flocculus by regulating inhibitory GABAergic synaptic transmission in floccular Purkinje cells and their terminals in the MVN. 73Glycine receptor (GlyR) mediates the downregulation of GABA A receptor and regulates the Purkinje cell's excitability by inhibiting GABAergic effects, 75 interestingly, the β subunit of GlyR was significantly increased from 8 hours to 3 days 81  Metabotropic glutamate receptors (mGluR) and ionotropic glutamate receptors also show significant changes during vestibular compensation.Ipsilateral flocculus mGluR2 is enhanced on the 1st day after UL, while mGluR7 remains unchanged. 59mGluR2 is present in the granular layer, and the presynaptic mGluR in the Golgi cell may be mGluR2, which further inhibits the inhibitory effect of the Golgi cell. 77The increase of mGluR2 in the ipsilateral flocculus eventually enhances long-term depression (LTD) in the flocculus, resulting in reduced inhibition of MVN. 59The expression of mGluR1α in unipolar brush cells increases 1 h after the UL, 5 upregulation of its expression enables unipolar brush cells to amplify upstream signals and transmit to granule cells.Ionotropic glutamate receptor KA2 increases at 6 hours, and reduces the inhibition of MVN. 78Downregulation of GluRδ2 receptors at the synapses of parallel fibers onto Purkinje cells also affects Purkinje cells regulation of MVN. 76sides the flocculus, the vestibulo-cerebellum also includes vermis, nodules/uvula (NU), 82 essential for static posture and eye movements.These regions have direct reciprocal projection with MVN neurons, 39,[83][84][85][86] and the Purkinje cells of NU are also innervated by vestibular ganglion neurons, 72,87 and tuned to vestibular translation stimuli via mossy and climbing fibers. 88,89In patients with vestibular neuritis, the local gray matter volume of the vermis decreased. 90ter the unilateral vestibular deficit, the glucose metabolism of the bilateral vermis was up-regulated. 91The expression of noradrenaline (NA) was up-regulated in the vermis of UL rats. 92These results suggest that the vermis may regulate vestibular compensation.
Studies of vestibular compensation have so far focused on the regulation of MVN by flocculus, although the vestibulo-cerebellum has long been thought to be related to MVN and to control motor balance, there have been few relevant studies.

| Tuberomammillary nucleus (TM) and histamine
Histamine is crucial for arousal and cognitive functions including learning and memory. 93The TM in the posterior hypothalamus is the only source of histamine neurons in the CNS, modulating the target cells through histamine receptors H1, H2 and H3, activating Gq, Gs and Gi, respectively. 49,50,94,95Some of the agonists and antagonists of these receptors are drugs for treating vestibular disorders, including unilateral vestibular damage. 49Unilateral vestibular injury reduces histamine staining of MVN and TM, 49 and the expression of histidine decarboxylase mRNA within the ipsilateral TM is upregulated, which might result from the direct projection from the vestibular nucleus to the posterior hypothalamus, and the upregulation of histidine decarboxylase might result in the rebalancing of the bilateral MVN via the projection from TM to MVN. 96,97 H1 receptor is expressed in GABAergic neurons in MVN, promoting the vestibular compensation via asymmetric activation of the commissural inhibitory system. 98H3 receptor is an auto-receptor on histaminergic and other types of terminals, its antagonists can also promote vestibular compensation, 99,100 for example, betahistine promotes vestibular compensation by acting on presynaptic histamine H3 and postsynaptic histamine H1 receptor. 49However, the mechanism of its function in vestibular compensation is largely unknown.

| Dorsal raphe nucleus and 5-HT
5-HT, also known as serotonin, is released by the dorsal raphe nucleus and regulates vestibular nuclei. 51,1015-HT receptors are all G protein-coupled receptors, some of these receptors including 0][51][52][53][54] The VN integrate inputs from various brain regions and neurotransmitters to regulate vestibular compensation.
3][104][105][106] The 5-HT1A and 5-HT1B receptors inhibit neuronal activity by suppressing adenylate cyclase and cyclic adenosine monophosphate (cAMP), and activate the Mitogenactivated protein kinase (MAPK) signaling pathway.The 5-HT2A receptor activates phospholipase C and PKC, and influences the MAPK pathway.Contrary to the 5-HT1 receptors, the 5-HT7 receptor activates neurons. 1079][110] This effect might be mediated by 5-HT1A. 1114][115] The patients of Meniere's disease with or without anxiety and depression can all be treated after giving SSRIs, suggesting that 5-HT may act directly on the vestibular system and treat Meniere's disease. 113,114wever, how 5-HT transmission is involved in vestibular compensation is poorly understood.

| Locus coeruleus (LC) and NA
LC in the pons of the brainstem is the main source of NA in the brain. 52,116,117LC plays an important role in controlling balance and motor functions, and its damage can lead to mild to moderate ataxia, often leaning towards the side of the lesion. 118NA is also involved in both the vestibulospinal reflex and vestibulo-ocular reflex, alterations in its levels can result in the deterioration of normal vestibular function. 119Retrograde tracing studies indicate a direct noradrenergic projection from the LC to the MVN. 120Impairment of this pathway seriously affects vestibular function and involves sensory mismatch during vertigo and motion sickness. 121Most MVN neurons are excited by NA via α1 and β receptors, while 82% of GABAergic neurons in MVN are activated by NA via α2 receptors and a few β receptors. 119,122,123During vestibular compensation, the expression of FOS protein increased significantly in LC, and noradrenergic neurons increased NA synthesis. 92,124,125However, injecting norepinephrine agonists into the ventricles result in decompensation behavioral effects. 126Therefore, the local effect of NA in each side of MVN needs further investigation.

| Pedunculopontine tegmental nucleus and acetylcholine
Cholinergic neurons in the brain are located in the basal forebrain and brainstem, 127 controlling vestibular function and processing sensory input information. 128The main group of acetylcholine neurons in the brainstem is located in the pedunculopontine tegmental nucleus (PPTg). 53They exert their influence by directly projecting to glutamatergic and GABAergic neurons within the vestibular nucleus. 38,39Despite the lack of conclusive data, PPTg is most likely the source of acetylcholine for MVN. 1291][132][133] After bilateral vestibular loss, acetylcholine neurons in PPTg are significantly activated. 129Local injection of acetylcholine or muscarinic receptor agonists into the vestibular nucleus resulted in postural deficits similar to those after unilateral vestibular injury. 134,135The downstream effects of cholinergic neurons can be excitatory or inhibitory according to the type of muscarinic acetylcholine receptors the cell expresses. 136It also has been reported that knockout of the α9 subunit of the nicotinic acetylcholine receptor inhibits the recovery of nystagmus and vestibular compensation. 137wever, the spectrum of acetylcholine receptor expression in MVN neurons remains largely unknown, especially the muscarine types of acetylcholine receptors.

| Lateral hypothalamic and orexin
Orexin is synthesized by neurons in the lateral hypothalamus (LH), [138][139][140] regulating feeding, energy homeostasis, sleep/wake cycle, and somatic motor control. 54,141LH orexin neurons project to and release orexin to MVN, and orexin 1 (OX1R) and 2 (OX2R) receptors are expressed in MVN. 142,143OX1R is coupled exclusively to the Gq subclass of heterotrimeric G-proteins, whereas OX2R can couple to Gq and Gi/Go. 144Orexin excites the GABA neurons via co-activation of OX1R and OX2R. 145,146After the bilateral vestibular injury, orexin labelling neurons and the orexin expression in LH significantly increase for 3 days, which might result from direct projection from MVN to LH. 86 The intracerebroventricular injection of the OX1R antagonist induces vestibular deficit behaviors, suggesting the importance of orexin in vestibular function. 147However, more research is needed to investigate orexin signaling in the vestibular system.

| VE S TIBUL AR CORTE X AND VE S TIBUL AR COMPENSATION
9][150][151] This distribution configuration of the cortical vestibular network, which offers resilience against cortical lesions like stroke and traumatic brain injury, may result from the integration of information from several sensory systems. 149However, whether this distributed cortical vestibular network could also contribute to the vestibular compensation after peripheral vestibular dysfunction is still unknown.This may be partly explained by the notable brain region differences between primates and other animal models, such as rodents.temporal area (hMST). 149Among these regions, OP2/PIVC and PIC/ VPS that located in the Sylvian fissure are densely interconnected, and OP2/PIVC is considered as the core of the cortical vestibular network. 152Nevertheless, the rodents lack the OP2/PIVC homologous brain region; vestibular signals instead activate the primary sensory cortical cortex, infralimbic, and cingulate cortices in the frontal regions. 153This may occur via the MVN ascending pathway, which goes to the bilateral sensorimotor cortices after projecting to the ipsilateral ventral posterior (VP) and ventral anterior (VA) thalamus. 154This indirect ascending pathway of MVN to the cortex via the thalamus is also conserved in primates. 155In response, the OP2/ PIVC in primates directly projects to the MVN, 148,156 regulating MVN-dependent vestibular reflexes, including the vestibulo-ocular, the vestibulo-spinal, and the optokinetic reflexes (Figure 4). 157,158 summary, the primate animal model will be crucial for the future investigation on the role of cortical vestibular network, especially OP2/PIVC, during vestibular compensation.

| CLINIC ALLY S IG NIFIC ANT
Currently, vestibular suppressants and antiemetic medications are the predominant treatments for vertigo. 1590][161] Through exercises, vestibular rehabilitation therapy facilitates the reorganization of the neural network by training the CNS and sensory system to accommodate vestibular impairment. 162,163Understanding the vestibular neural network will facilitate the discovery of novel drug targets and rehabilitation strategies. 164

| FUTURE DIREC TIONS
The bilateral vestibular nuclei are the essential hub for vestibular compensation, the commissural inhibitory system between the bilateral vestibular nuclei is crucial for the activity balance in the two nuclei and functional recovery from unilateral vestibular damage.
Recent advances in neural networks revealed several neurotransmitters, including glutamatergic, GABAergic, histaminergic, cholinergic, serotonergic, and adrenergic neurons are involved vestibular compensation, however, more research is needed to elucidate their mechanism in vestibular compensation, which will greatly facilitate developing new treatment strategies for vestibular system-related diseases. Why in MVN and the flocculus, which might activate ipsilateral MVN neurons via disinhibition.Protein Kinase C (PKC) isoforms display an asymmetric distribution in Purkinje cells after Unilateral labyrinthectomy (UL) and the Purkinje cells LTD was dependent on PKC activity.The recovery of PKC expression was synchronized with the disappearance of spontaneous nystagmus in rats, 74 suggesting the importance of PKC in vestibular compensation.
These inputs include 5-HT from the DR, histamine from the TM, Ach from the PPTg, orexin from the LH, NA from the LC, and GABA input mainly from the cerebellum, particularly the flocculus.The homeostasis of these neurotransmitters is essential for the complete function of the VN.The upper figure is of a mouse, and these data are derived from mice, indicated by the solid line.The lower figure is of a human, inferred from the results of the mouse, represented by the dashed line.DR, Dorsal raphe nucleus; LC, Locus coeruleus; LH, Lateral hypothalamic; PPTg, Pedunculopontine tegmental nucleus; TM, Tuberomammillary nucleus; VN, Vestibular Nuclei.

| 9 of 14 WANG
et al.The cortical vestibular network in human is composed of at least 10 cortical regions, including area 2v and 3a regions in the primary somatosensory cortex, area 7 of the parietal cortex, the premotor area, cingulate sulcus visual (CSv), ventral intraparietal area (VIP), parietal operculum cortex 2 (OP2)/parieto-insular vestibular cortex (PIVC), posterior insular cortex (PIC)/visual posterior sylvian area (VPS), supplementary motor area (SMA) and human medial superior 158 connection between vestibular cortex and MVN in humans.158Vestibularcortex projects directly to MVN, and through the vestibularthalamic-cortex connection, MVN indirectly projects to the cortical vestibular network.InC, Interstitial nucleus of Cajal; MVN, Medial Vestibular Nucleus; OP2, Parietal operculum cortex 2; PIC, Posterior insular cortex.regulatory mechanisms of the vestibular neural networks, offering a novel approach, establishing a neuropharmacological basis for clinical drug therapies and tailored rehabilitation strategies for vertigo patients.With a clear vision and innovative spirit, 'Lead the way, carve a unique path.' do patients with the same vertigo condition experience varying outcomes in rehabilitation and prognosis?Vertigo and other conditions can exacerbate each other, creating a vicious cycle that frequently results in diminished treatment effectiveness and unfavorable outcomes.For the challenges and poor prognoses associated with vertigo treatment, this review delves into the intricate F I G U R E 4